What's the best way of implementing a messaging queue table in mysql

Your dequeue could be more concise. Rather than relying on the transaction rollback, you could do it in one atomic statement without an explicit transaction:

UPDATE jobs SET process_id = ? WHERE process_id IS NULL ORDER BY ID ASC LIMIT 1;

Then you can pull jobs with (brackets [] mean optional, depending on your particulars):

SELECT * FROM jobs WHERE process_id = ? [ORDER BY ID LIMIT 1];

Brian Aker talked about a queue engine a while ago. There's been talk about a SELECT table FROM DELETE syntax, too.

If you're not worried about throughput, you can always use SELECT GET_LOCK() as a mutex. For example:

SELECT GET_LOCK('READQUEUE');
SELECT * FROM jobs;
DELETE FROM JOBS WHERE ID = ?;
SELECT RELEASE_LOCK('READQUEUE');

And if you want to get really fancy, wrap it in a stored procedure.

I've built a few message queuing systems and I'm not certain what type of message you're referring to, but in the case of the dequeuing (is that a word?) I've done the same thing you've done. Your method looks simple, clean and solid. Not that my work is the best, but it's proven very effective for large-monitoring for many sites. (error logging, mass email marketing campaigns, social networking notices)

My vote: no worries!